[Objective] The spatio-temporal variations of a typical herbicide (atrazine, ATR) and its degradation products[deethylatrazine (DEA) and deisopropylatrazine (DIA)] during the crop growing season (June to September) was investigated in order to provide scientific supports for management, usage, and protection of herbicides in a small agricultural watershed.[Methods] Eighteen sampling sites were designated along the Baima River, and the spatio-temporal dynamics of pollutants were obtained through the combined methods of field monitoring and lab analysis. Ecological risk was evaluated by the risk of herbicides quotient method.[Results] ① ATR and its metabolites were detected in all of the collected water samples. ATR detection rates in surface water samples in June and July were higher than those in August and September. However, the detection rates of metabolites including DEA and DIA were low in the first two months and increased in the later two months. ② The spatial distribution of herbicide pollution in the Baima River showed that the maximum values of pollutants mainly occurred in the upper and middle reaches of the river. The concentration levels in the lower reaches were relatively low, which may be attributed to the wide river channel, large water flow, and strong dilution effect in the lower reaches of the river. ③ The risk levels of ATR, DEA, and DIA for the entire river were all at the moderate level due to low water load and insufficient dilution capacity of the river.[Conclusion] Compared with larger watersheds, the Baima River, as a small tributary of typical agricultural watersheds, has small water volume and insufficient dilution capacity that results in higher ecological pollution risks than many larger basins.